The idea of efficiency of an exchanger differs from the concept of efficiency loss. Efficiency of an exchanger is its ability to exchange the heat in to the heat out - as in 100.000 btuh in at 180F to 70,000 btuh out at 150F.

There is no loss during the exchange, other than that lost from the surface of the exchanger and the tubing to the air around it, which is a small surface loss.

You just want to make sure that the amount you get out of at at what temperature to be used by the radiators or hot water - can be supplied by the exchanger at a reasonable temperature coming from the boiler. That is in the spec chart for the exchanger.